The present invention relates to method and a system for changing a vehicle's trajectory, wherein the vehicle comprises a steering arrangement comprising a manual steering device, at least one pair of ground engaging members and a mechanical interconnection between the manual steering device and said ground engaging members.
A number of active safety functions aim at changing the vehicles future trajectory, for reasons such as avoiding a collision, a roadway departure or obtaining a proper lane position. A change in trajectory can be done through a change in steering wheel angle, which is exactly what the driver does while driving the vehicle.
Thus, the vehicle's trajectory may be changed due to an undesired situation such as a departure from a desired future trajectory of the vehicle. In other words, the invention is applicable for so-called lane keeping of a vehicle during operation. A current driving scenario that represents an unintentional lane departure may be determined based on the vehicle's position, direction and/or orientation with respect to a traffic lane (r road edge). Further, there are systems known, which are configured for monitoring the course of the traffic lane ahead of the vehicle, such as by monitoring lane markings using a vision system. The lane keeping support system is preferably configured to provide such guiding force only in situations in which the supply of such guiding force to the steering device is deemed to be appropriate after analysis of all input data, such as the course of the traffic lane ahead of the vehicle, further vehicles on the road and a predicted driving behaviour of the vehicle.
The guiding force exerted onto the steering device is resistive if counteracting the force applied by the driver onto the steering device, or supportive if acting in the same direction as the force applied by the driver onto the steering device, thus for instance reducing the effect of e.g. frictional forces acting on the wheels and the like which are experienced by the driver as resistance when operating the steering, device. The steering device is normally formed by a conventional steering wheel in the case of a vehicle. However, the invention is applicable to other steering devices, such as a joystick, a sliding nipple or any other suitable steering device for steering the vehicle. For instance, in the case that the steering device is a steering wheel, the guiding force will appear as a guiding torque exerted onto the steering wheel.
In many countries/regions there are legal requirements limiting the allowable guiding force to be applied to the steering device. According to a known method, the guiding torque applied to the steering wheel is automatically limited to the allowed limit during an intervention. However, such automatic limitation may lead to that the intervention is unsuccessful since the intervention could not be carried out to the desired extent.
It is desirable to achieve a method for changing a vehicle's trajectory which creates conditions for a further improved safety during operation, especially in case there is a predefined limit for the amount of a steering device guiding force.
A method according to an aspect of the invention includes applying a braking force to at least one of said ground engaging members so that the vehicle's trajectory is changed, and simultaneously suppressing steering device disturbances resulting from the mechanical interconnection.
The term “ground engaging members” comprises wheels fitted with tyres, but may also cover other types of ground engaging members, such as caterpillar tracks.
The braking force is preferably applied by means of a differential braking force to said pair of ground engaging members.
Thus, the vehicles trajectory can be changed by differential braking,—where a longitudinal force is generated by applying a brake pressure to a single wheel or a wheel pair (such as front and rear at the same side). The resulting longitudinal force generates a torque around the vehicles centre of gravity, thereby changing the vehicle trajectory.
A problem with prior art solutions for differential braking on the front axle is that a disturbing torque is introduced into the steering system which results in a steering wheel angle. As a consequence, a front axle sideslip angle is introduced, which results in a lateral force. This force can act so that the resulting torques of lateral and longitudinal force around the vehicle's centre of gravity counteract or act in the same direction. Which of these two alternatives that is the case depend on the actual design of the front suspension. In any case, the exact amount of the achieved steering wheel angle is dependent whether the driver has the hands on the wheel or not, and which steering wheel angle that is allowed by the driver. In other words, the loop gain of the transfer function between lateral acceleration (or yaw rate) and brake pressure for wheel brake torque) is dependent on both the actual vehicle front suspension and the driver.
By the inventive step of simultaneously suppressing steering device disturbances resulting from the mechanical interconnection, the problem of the disturbing torque introduced into the steering system is relieved.
According to an example embodiment, the method comprises the step of receiving a signal indicative of a current driving scenario, determining if it is desired to change the vehicle's trajectory by braking said at least one ground engaging member based on the driving scenario signal in order to avoid an undesired situation and if so automatically applying, the braking force to said at least one ground engaging member. Thus, changing the vehicle's trajectory can be performed also in additional ways to braking said at least one ground engaging member. One such additional way is to charge the vehicle's trajectory by steering via the steering arrangement. For example a guiding force is applied to the steering device for changing the vehicle's trajectory. Preferably, the guiding force is applied during a driver steering operation. Preferably, the guiding force applied is only supportive, i.e. it is limited to such an extent that the driver still has full authority to steer the vehicle. However, the system may be configured to take control of the vehicle and in the case of lane keeping, return it to a safe position in the original lane.
Preferably, the undesired situation represents a predicted departure from a predicted desired future trajectory of the vehicle.
According to a further example embodiment, the method comprises the step of suppressing said steering device disturbances resulting from the mechanical interconnection by decoupling a driver steering feel from the influence of the mechanical connection. Thus, this embodiment creates conditions for decoupling the hardware (mechanical connection) from the steering feel. In other words, the embodiment creates conditions for an application-independent (hardware-independent) steering feel.
According to a further example embodiment, the method comprises the step of suppressing said steering device disturbances resulting from the mechanical interconnection by receiving a signal indicative of a steering angle and/or steering, torque in the steering arrangement, determining if the steering angle will result in steering device disturbances, determining if it is desired to apply a force counteracting the steering device disturbances resulting from the steering angle and/or steering torque and if so generating a corresponding signal to an actuator arranged to apply such a counteracting force to the steering arrangement.
Preferably, a steering torque is determined by measuring the twist of a torsion bar in the steering arrangement. More precisely, a first angular sensor is arranged at a first end of the torsion bar and a second angular sensor is arranged at a second end of the torsion bar (opposite the first end). The steering torque can be determined based on the relative angular movement (twist) of the torsion bar and the stiffness of the torsion bar. According to an alternative, one or several strain gauges may be used.
Such a method may be performed via an Electronic Power Assisted System (EPAS) comprising a controlling function, below referred to as a reference generator, which is configured to determine a desired torque to be applied, to the steering wheel in order to provide the driver with a desired steering feel. In other words, the reference generator describes a nominal vehicle.
Thus, there is a mechanical connection between the steering device and the ground but the inherent steering feel resulting from the mechanical connection during, operation is eliminated or at least suppressed. In other words, the t guiding force is continuously determined during operation so that the driver experiences a desired feel in the steering device instead of the inherent steering feel resulting from the mechanical connection.
When a reference generator is used for steering system control, then the disturbance torque is automatically compensated for, and thus no front axle slip angle is produced, and thus no lateral force. The influence of both suspension geometry and driver is thus effectively removed from the loop. The braking function therefore becomes a much more predictable process.
Therefore, according to a further example embodiment, the method comprises the step of providing the driver of the vehicle with a desired steering feel based on a determined desired guiding force, which is applied to the steering device. Preferably, the method comprises the step of applying the determined guiding force to the steering device via said actuator. From an implementation point-of-view it is wise to add guiding torque via the actuator if the actual torque is lower than the desired torque for a determined steering feel and to cancel guiding torque via the actuator if the actual torque is higher than the desired torque for the determined steering feel.
According to a further example embodiment, the method comprises the step of receiving at least one signal indicative of a vehicle state and determining the guiding force based on said at least one vehicle state signal. Preferably, the method comprises the step of determining a value and direction of the guiding force for changing the vehicle's future trajectory. The value of the guiding force may be determined for assisting the driver in changing the vehicle's future trajectory.
According to a further example embodiment, the method comprises the step of comparing the determined guiding force with a limit value, and if the determined guiding force exceeds the limit value applying, the braking force to said at least one ground engaging members so that the vehicle's trajectory is changed.
The limit value can represent a maximum allowable torque, which the EPAS can add due to legal requirements or internal safety requirements. Preferably, the changing of the vehicle's trajectory is performed via the steering arrangement and only if such a limit is reached the brakes are used to increase the torque around the vehicles centre of gravity, thereby contributing to the change the vehicle trajectory or alternatively completely taking over the steering.
Let's say we would like to steer the vehicle to the right and the suspension geometry is as in one type of truck. We would then brake the wheels on the right hand side of the vehicle. We would get a torque around the centre of gravity turning the vehicle in the clockwise direction. Without the reference generator function the front right wheel would get a disturbance angle to the right. With the reference generator function this disturbance angle would be removed leading to a much more predictable steering by braking.
According, to a further example embodiment, the method comprises the step of limiting the guiding force applied to the steering device to the limit value or below. Thus, in addition to the steering by braking, a steering effect is achieved by means of the steering arrangement.
According to a further example embodiment, the method comprises the step of determining the guiding force based on at least one steering device guiding force operation model. Preferably, the method comprises the steps of continuously during operation determining the guiding force, continuously applying the determined guiding, force to the steering device, and modifying/interrupting the determined guiding force if the limit value on steering device torque is reached.
Preferably, said at least one guiding force operation model comprises at least one desired steering characteristic parameter. Preferably, said at least one desired steering characteristic parameter comprises at least one of damping, of steering device movements, tire friction, self alignment of the steering device to a neutral position and friction in a mechanical connection between the steering device and the wheels. Especially, said at least one desired steering characteristic parameter comprises at least lateral acceleration and/or yaw, rate and the method comprises the step of modifying and/or canceling the effect of the lateral acceleration and/or yaw rate.
A steering by braking will give a lateral acceleration (or yaw rate) on the vehicle which will give a deviation between the real vehicle behaviour and the behaviour estimated by the vehicle model in the reference generator function. >>This deviation will give a torque in>the steering wheel that will work against the steering by braking. To avoid this the measured lateral acceleration (or yaw rate) needs to be decoupled during a steering, by braking. Alternatively this torque in the steering wheel can be compensated for since we know how much torque around the centre of gravity the steering by braking will give.
The desired lateral acceleration (yaw rate) from steering by braking will give a desired torque around the vehicles centre of gravity by using the estimated Inertia of the vehicle. From the desired torque around the vehicles centre of gravity brake forces can be calculated if the track width is known.
Further, to be able to know how much torque around, the vehicles centre of gravity the brakes will contribute with ifs advantageous to have a good estimate of the brake factors of the brakes. The brake factor relates the brake pressure to the brake torque and can vary due to e.g. speed, brake pressure, temperature and contamination.
It is also desirable to achieve a system for changing a vehicle's trajectory which creates conditions for a further improved safety during operation, especially in case there is a predefined limit for the amount of the guiding force.
A system for changing a vehicle's trajectory according, to an aspect of the invention is provided, wherein the vehicle comprises a steering arrangement comprising a manual steering device, at least one pair of ground engaging members and a mechanical interconnection therebetween characterized in that the system comprises an arrangement for applying a braking force to at least one of said ground engaging members so that the vehicle's trajectory is changed, and an arrangement for suppressing steering device disturbances resulting from the mechanical interconnection.
According to an example embodiment, said arrangement for suppressing steering device disturbances comprises means for detecting a steering angle and/or steering torque in the steering arrangement, means for determining if the steering angle and/or steering torque will result in steering device disturbances and generating a corresponding signal and an actuator arranged to receive the signal and responsively apply a force counteracting the disturbances to the steering arrangement. Preferably, a delivered steering device guiding force is measured and compared with an estimated desired steering device guiding force, wherein the delivered steering device guiding, force is adapted by use of a feedback controller to be substantially the same as the desired steering device guiding force through adapting the amount of said guiding force.
According to a further example embodiment, said system comprises means for receiving a signal indicative of a current driving scenario, determining if it is desired to change the vehicle's trajectory by braking based on the driving scenario signal and generating a corresponding braking force signal to said means for applying a braking force.
According to a further example embodiment, said means for determining if it is desired to change the vehicle's trajectory is configured to generate a signal to a means for controlling the steering arrangement.
According to a further example embodiment, said system comprises means for providing the driver of the vehicle with a desired steering feel based on a determined desired guiding force.
Further preferred embodiment and advantages thereof emerge from the description below, the figures and the claims.